In recent years, so-called “hybrid vehicles”, each including an engine and a motor, have been popularized. Motors have advantages over engines, because motors achieve faster torque response, higher torque controllability, higher energy conversion efficiency, and less noise and vibration than engines. On the other hand, engines have their own advantages, because engines use petroleum fuel with high energy density and achieve a long cruising distance. Various techniques for hybrid vehicles have been and are being developed to take full advantage of an engine and a motor that are used in combination as a hybrid power source.
There are various types of hybrid vehicles, examples of which include series, parallel, and series-parallel hybrid vehicles, which are classified according to the method of coupling between a motor and an engine. A series hybrid vehicle is driven by a drive motor with an engine mechanically disengaged from wheels. Power is transmitted in series from the engine to a power generation motor, to a secondary battery, to the drive motor, and then to the wheels in this order. A parallel hybrid vehicle is driven by mechanically connecting a drive motor and an engine to wheels. Power is transmitted in parallel from the engine to a transmission and then to the wheels and from a secondary battery to the drive motor and then to the wheels. A series-parallel hybrid vehicle is a combination of the series hybrid type and the parallel hybrid type. Examples of series-parallel hybrid vehicles include a type in which power is changed by engagement and disengagement of a clutch and a power-split type in which the power distribution ratio is changed using a planetary gear.
Patent Document 1 discloses a parallel hybrid vehicle which includes an engine driving either one pair of wheels (namely, front wheels or rear wheels) and a motor driving the other pair, and in which a motor speed reduction gear transmitting the driving force of the motor to the wheels includes a switching mechanism engaging and disengaging a sleeve with/from a clutch gear.